On the terrace, a Sultana-designed sofa clad in a Scalamandré fabric is grouped with two Fredrikson Stallard tables and an André Dubreuil chair.

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Designer Francis Sultana and architect Thomas Croft renovated a triplex apartment in a 19th-century London mansion for its art-collecting owner and her family. In the drawing room, a Steinway piano is positioned between a light sculpture by Elizabeth Garouste and Mattia Bonetti and a cabinet by Bonetti. Over the cabinet hangs a sculpture by Thomas Houseago, while a painting by Keith Coventry is displayed above a Martin Szekely console nearby. The clear cylindrical side table is also by Szekely, and the crocodile stool is by Claude Lalanne.

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A sinuous mirrored table by Ron Arad separates twin sofas designed by Sultana; the white geometric side table is by Bonetti, and the carpet is by Fort Street Studio.

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At one end of the drawing room, a Jean Royère sofa is grouped with Bonetti polished-steel side tables and a Charlotte Perriand bench with a cushion covered in an Edelman Leather cowhide; the large framed painting near the window is by Anselm Reyle, the photograph on the mantel is a piece by Marlo Pascual, and the pair of table lamps is by Garouste and Bonetti.

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A Fredrikson Stallard light fixture hangs over the first-floor dining table, which is topped with a trio of Ai Weiwei vases and lined with Bonetti chairs.

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At one end of the first-floor dining room, Vladimir Kagan sofas frame a Willy Rizzo table; the photograph above the fireplace is by Wolfgang Tillmans, and the sculptures are by Rebecca Warren.

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In the garden-level dining room, a painting by Fiona Rae overlooks a Szekely table and chairs.

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The garden-level kitchen’s cabinetry is by Minotti Cucine, with a cooktop by Wolf and barstools by Sultana; the sculpture at right is by Nicole Wermers.

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On display at the top of the stairway are a wall-size work by Keith Tyson and a Cerith Wyn Evans light installation.

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A two-panel painting by John Stezaker graces the partition that conceals a bath within the master bedroom; the bed and chaise longues were designed by Sultana.

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The master bedroom’s closet, customized by Sultana, has palladium-leaf doors; the carpet is by Fort Street Studio.

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In the master bath, a work by Aaron Curry is reflected in the mirror; the vanity is by Bonetti, and the sink fittings are by Grohe.

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Mirrors by FontanaArte and pendant lights by Garouste flank a Nigel Cooke painting above a guest-room fireplace; the chairs are vintage Jansen, and the carpet is by Fort Street Studio.

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A tub by the Water Monopoly is matched with a Corian vanity by DuPont in a second-floor bath; the sink and tub fittings are by Grohe, and the floor is statuary marble.

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The children’s bed- and-library unit was designed by Croft; a Campana brothers stuffed-animal chair sits on a Fort Street Studio carpet, and the artwork at right is a photogram by Adam Fuss.